1. Field of the Invention
The present invention relates to a wireless communication system and method for controlling antenna downtilt/uptilt.
2. Description of Related Art
Conventional wireless communication systems include a plurality of cell sites, each having a base station sending and receiving signals over one or more associated antennas or antenna modules. The antenna module usually includes at least one receive and one transmit antenna, but could use a single antenna for both the transmit and receive functions. The radiation pattern (particularly, the main lobe) of, for example, a transmitting antenna at a cell site may be tilted from a horizontal reference of the antenna by a certain angle. This angle is referred to as the downtilt angle of the antenna, and is measured to be positive from the horizontal reference of the antenna towards the ground. Accordingly, an antenna with a downtilt angle of 10 degrees tilts towards the ground more than an antenna with a downtilt angle of 5 degrees.
Each antenna has a coverage area, which is a geographic area in which a mobile terminal will communicate with a base station associated with the antenna. The extent of an antenna""s coverage area is affected by its downtilt angle and the downtilt angles of surrounding, but not necessarily adjacent, antennas.
Conventionally, the downtilt angles of the antennas in wireless communication systems are set at the time of system installation according to predetermined downtilt angles. Installation workers climb up each antenna tower or support (e.g., a building), supporting antennas in the system, and manually fix the downtilt angle of each antenna according to the predetermined values. If the downtilt angle needs to be changed after the network installation, the worker has to once again climb up the antenna tower to manually adjust the downtilt angle of the antenna. While it may be practical to make adjustments of the wireless communication system in this manner to small portions of the system, making adjustments is cumbersome, time consuming, costly and potentially dangerous since it requires a worker to go up the antenna tower and adjust the downtilt angle of the antenna. The difficulty, cost and complexity, however, increases as the number of antennas requiring downtilt angle changes increases. Also, it is impractical to make downtilt angle adjustments based on short term events, such as changes in the time of day (e.g., mobile terminal traffic in a coverage area for a business complex will be greater during business hours), and most long term events (e.g., a change in seasons wherein foliage affects signal-to-noise ratio).
Because the downtilt angles of antennas in a wireless communication system directly effect the quality of system performance, a demand exists for a simple, easy and cost effective manner in which to change the downtilt angles of the antennas in a wireless communication system to improve system performance. Typically, operators monitor the quality of their system by taking operational measurements indicative thereof. These operational measurements include, but are not limited to, co-channel interference (i.e., interference between two signals using the same channel frequency), signal-to-noise plus interference ratios within coverage areas, bit error rates within coverage areas, call blocking rates (e.g., the ratio of (1) the number of mobile terminals in a coverage area having their call requests denied by the base station because of insufficient resources at the base station which are dedicated to (2) the antenna module for that coverage area to the number of mobile terminals requesting calls in the coverage area) within coverage areas, etc. For example, signal strength interference measurements between two coverage areas can indicate an amount by which signals transmitted by adjacent antennas overlap; and therefore, provide an indicator as to the quality of hand-offs between coverage areas for these adjacent antennas. As another example, high call block rates can indicate unacceptable levels at which customers (i.e., mobile terminal users) are denied service and/or an overload condition. Typically, when the call blocking rate or other measure of load on a base station is greater than a predetermined threshold, the base station serving that coverage area or the coverage area itself is said to be overloaded.
Some of the operational measurements are made by one or more test receivers at known measurement locations within the wireless communication system, and making the operational measurements using the test receiver. Other operational measurements, such as call blocking rates, are made as part of system operation. The changes in operational measurements over time may reflect changes within the coverage area such as a population increase, addition of a new structure (e.g., a building), etc that affect the quality of system performance. Based on the operational measurements, changes may be made to the wireless communication system to improve the quality of system performance.
For instance, when a problem, such as poor coverage (e.g., low signal-to-noise ratio for signals received in a coverage area), is indicated by the operational measurements, the signal strength of signals transmitted by the antenna for the problem coverage area may be changed and the signal strength of signal transmitted by one or more antennas for coverage areas adjacent thereto may be changed until the operational measurements show acceptable coverage.
A demand, however, exists for greater freedom in addressing quality of system performance concerns. Namely, a demand exists for improved, alternative or additional methods of addressing quality of system performance concerns. A simple, easy, and cost effective manner of adjusting the downtilt angles of antennas in a wireless communication system would facilitate meeting such demands.
The present invention provides a wireless communication system including antennas having electrically controllable downtilt angles and downtilt controllers associated with each antenna. The downtilt controllers receive instructions from a main controller, and adjust the downtilt angles of the associated antennas in accordance with the received instructions. From the main controller, an operator can effect changes that vary from system wide changes in the downtilt angles of the antennas in the system to changes in the downtilt angle of a single antenna. From the base station associated with an antenna, an operator on the ground can effect changes in the downtilt angle of the associated antenna. Making changes in the downtilt angle, whether from the base station or the main controller, using the present invention avoids the costly and dangerous process of climbing a tower or other support structure to manually adjust the antenna""s downtilt angle.
Because the process of changing downtilt angles is so simple with the present invention, the present invention allows adaptive control of the downtilt angles to address issues of quality in the system, such as hand-off quality (e.g., signal overlap), denial of service (e.g., load), co-channel interference, signal-to-noise plus interference ratios, bit error rate, etc, even during system performance. Furthermore, the system according to the present invention allows automating the process or portions of the process for addressing these quality issues.